(1) Field of the Invention
This invention relates to a receiving unit, receiving method, and semiconductor device and, more particularly, to a receiving unit and receiving method for receiving and demodulating signals sent from a base station and a semiconductor device for processing signals sent from a base station.
(2) Description of the Related Art
With code division multiple access (CDMA) communication, information sent via a plurality of channels or information sent from users is multiplexed with a spreading code and is transmitted via a radio line.
Random changes in amplitude and phase and fading with a maximum frequency which depends on the speed of a mobile station and the frequency of a carrier will occur in mobile communication. As a result, compared with radio communication between fixed stations, it is difficult to perform stable receiving.
Spread spectrum communication will reduce degradation in communication quality caused by such frequency selective fading. With this spread spectrum communication, signals in a narrow band are spread in a wide band and are sent. Therefore, even if receiving field strength weakens at a specific frequency, information can be restored from other bands.
Moreover, it has been shown that if fading occurs in mobile communication, delayed waves caused by buildings and terrain will create a multipath fading environment.
With direct-sequence spread spectrum communication, delayed waves will be interference waves for a spreading code, so receiving characteristics will be degraded. RAKE receiving is known as a method for improving characteristics by making use of delayed waves.
With the RAKE receiving, despreading is performed on each delayed wave which arrives via each of multipaths, the delay time of delayed waves is matched, weighting is performed on the delayed waves according to their received levels, and the delayed waves are added together. This can minimize the influence of multipaths.
Multipaths are searched for by a path search block which includes a matched filter section, delay profile integration section, and path selection section. FIG. 11 is a block diagram of a conventional receiving unit based on RAKE receiving.
As shown in FIG. 11, a conventional receiving unit comprises an antenna 10, a receiving section 11, a quadrature detection section 12, analog-to-digital (A/D) conversion sections 13 and 14, a path search section 15, and a RAKE combining/demodulation section 16.
The antenna 10 acquires electronic waves sent from a base station.
The receiving section 11 converts electronic waves acquired by the antenna 10 into radio frequency (RF) signals, converts the RF signals into intermediate frequency (IF) signals, and outputs the IF signals.
The quadrature detection section 12 performs quadrature detection on IF signals output from the receiving section 11 to separate and demodulate I channel (Ich) signals and Qch signals.
The A/D conversion sections 13 and 14 convert Ich signals and Qch signals, respectively, output from the quadrature detection section 12 into digital signals and output them.
The path search section 15 receives digital signals output from the A/D conversion sections 13 and 14, searches them for multipaths, and outputs the timing of each path.
The RAKE combining/demodulation section 16 refers to timing signals supplied from the path search section 15, performs a despreading process on Ich signals and Qch signals, which are output from the A/D conversion sections 13 and 14 respectively, according to paths, recovers the original data from I symbol data and Q symbol data obtained by the despreading, and combines and outputs recovered results.
FIG. 12 is a view showing the detailed structure of the path search section 15. As shown in FIG. 12, the path search section 15 includes a matched filter 30, delay profile integration section 31, and path selection section 32.
The matched filter 30 consists of a 256-tap matched filter and calculates and outputs the auto-correlation values of desired signals included in Ich signals and Qch signals.
The delay profile integration section 31 includes a power value calculation section 31a and memory 31b and calculates and outputs power values by integrating output from the matched filter 30 and calculating geometric means.
The power value calculation section 31a calculates power values by integrating input Ich signals and Qch signals by the slot and calculating geometric means, integrates the power values by the frame (“frame” is greater than a “slot”), and outputs the results.
The memory 31b temporarily stores data to be used by the power value calculation section 31a for performing an operation.
The path selection section 32 refers to data output from the delay profile integration section 31, selects n paths in descending order of power of received signal, and outputs information indicative of their timing as effective multipath information.
Now, operation in the above conventional receiving unit will be described.
The antenna 10 acquires electronic waves sent from a base station and supplies them to the receiving section 11.
The receiving section 11 converts the electronic waves acquired by the antenna 10 into RF signals, converts the RF signals into IF signals, and outputs the IF signals.
The quadrature detection section 12 multiplies the signals output from the receiving section 11 and a sine wave together to generate Ich signals, multiplies the signals output from the receiving section 11 and a cosine wave together to generate Qch signals, and outputs the Ich and Qch signals.
The A/D conversion section 13 converts the Ich signals (analog signals) output from the quadrature detection section 12 into digital signals and output them.
The A/D conversion section 14 converts the Qch signals (analog signals) output from the quadrature detection section 12 into digital signals and output them.
By performing a predetermined process on the Ich and Qch signals, the path search section 15 selects n paths in descending order of power and outputs the timing of each path. That is to say, as shown in FIG. 13, the received levels of signals sent from a sending unit differ among different multipaths. Moreover, the time when these signals arrive at the receiving unit also differs among the different paths. The matched filter 30 in the path search section 15 receives the Ich and Qch signals, multiplies the Ich signals and a predetermined despreading code together, and multiplies the Qch signals and a predetermined despreading code together. By doing so, the matched filter 30 extracts and outputs signals for a user's channel included in the Ich and Qch signals.
The power value calculation section 31a in the delay profile integration section 31 first integrates data output from the matched filter 30 by the slot. Then the power value calculation section 31a calculates the geometric means of data obtained by the integration by the slot to obtain power values. In this case, there are two kinds of signals: the Ich signals and Qch signals. Therefore, the power value calculation section 31a obtains power values by calculating, for example, (I+jQ)×(I−jQ)=I×I+Q×Q. Then the power value calculation section 31a integrates the power values by the frame and outputs the results.
The path selection section 32 refers to the data output from the delay profile integration section 31, selects n paths from among a plurality of paths in descending order of power, and outputs information indicative of their timing as effective multipath information.
As shown in FIG. 14, a fixed range with the maximum multipath detected the last time as a center which has a width of a total of 256 chips is set and multipaths are detected in this range. This setting is based on data regarding terrain which will generate the strongest multipaths.
By the way, the precision of detection is improved by oversampling received signals by the chip. For example, if an oversampling rate and the number of received bits for the I channel and Q channel are 4, 6, and 6 respectively, then 256×4×6=1024×6 bits for both the I and Q channels.
The matched filter 30 usually includes shift registers and addition trees. Therefore, if such oversampling is performed, a vast number of shift registers and addition trees corresponding to the above number will operate in synchronization with a basic clock. As a result, a large amount of power will be consumed.
In addition, the delay profile integration section 31 will generate data corresponding to 1024 samples for one slot. To store all these pieces of data, memory areas corresponding to 1024 words are needed in the memory 31b. 
Furthermore, the path selection section 32 needs to select, for example, three multipaths in descending order of power from data corresponding to 1024 samples, which are output from the delay profile integration section 31, and to output them.
As stated above, conventional receiving units have detected multipaths on the basis of data regarding terrain which will generate the strongest multipaths. Therefore, when these receiving units are used in an ordinary environment, paths will be detected in an unnecessarily wide range.
As a result, a vast amount of power will be consumed. Sufficient communication time therefore cannot be secured especially in the case of battery capacity being limited (in mobile communication, for example).